The present invention relates to digital frequency modulators for the Sequentiel Couleur Avec Memoire (SECAM) color television standard generally and, more particularly, to a method and/or architecture for a frequency modulated subcarrier using a single low frequency pre-emphasis filter and P/Q ratio counter.
Sequentiel Couleur Avec Memoire (SECAM) is a video standard that defines a 50 field per second, interlaced 625 line video format. Color information is conveyed with the use of frequency modulation (FM). Red and blue color difference information is frequency modulated on alternating lines (i.e., one color difference signal per line).
Previous digital SECAM encoder designs, such as the design described on pages 173-179 of xe2x80x9cVideo Demystifiedxe2x80x9d by Keith Jack, use two low frequency pre-emphasis filters and P/Q ratio counters. The previous designs implement a first filter and ratio counter pair for red color difference modulation and a second filter and ratio counter pair for blue color difference modulation.
The vast majority of SECAM encoders have been designed using analog techniques. The conventional analog designs comprise two reference frequency generators with associated phase control loops. Such analog designs are tied very closely to a specific process technology and cannot be easily ported to other processes and/or integrated with other system functionality on a single device.
The present invention concerns an apparatus comprising a first circuit and a second circuit. The first circuit may be configured to present a first signal and a second signal in response to (i) a select signal, (ii) a first difference signal and (iii) a second difference signal. The second circuit may be configured to present a modulated output signal in response to the first signal, the second signal and a control signal. The first circuit may alternately operate on one of either (i) the first difference signal or (ii) the second difference signal.
The objects, features and advantages of the present invention include providing a method and/or architecture for modulating a chroma video component (e.g., in a SECAM signal) that may (i) generate a modulated subcarrier for color difference signals with a single filter and ratio counter combination, (ii) allow color difference signals to be filtered and modulated on alternating lines, (iii) set a current subcarrier starting phase for each video line, (iv) initialize a low frequency pre-emphasis filter for each video line, (v) be implemented using fewer transistors and reduced die space, (vi) provide a reduced power dissipation and/or (vii) provide a modulated subcarrier with extremely small frequency errors.